1. Field of the Invention
The present invention generally relates to a method and apparatus for optical phase modulation for use in the field of optical telecommunication systems, in particular to a method and apparatus using the multi-level phase shift key (MPSK) modulation technique.
2. Description of the Related Art
Currently deployed optical telecommunication systems make mostly use of binary on-off key (OOK) modulation format with direct detection at the receiver (OOK/DD). In the art, OOK/DD is also referred to as (binary) intensity modulation with direct detection (IM/DD). By modulating the optical intensity, a binary-digital information signal is encoded in a corresponding binary-digital optical signal consisting of a stream of optical pulses. Usually, in OOK/DD format a logical binary digit (bit) “1” is associated to a first level of optical intensity in a time slot of the optical stream, while a logical bit “0” is associated to a second level of optical intensity different from the first level. The time slot corresponding to a single bit (both 1 or 0) is known as bit-period T [s] and the optical pulse stream is characterized by an optical rate B=1/T [s−1]. In OOK/DD, the rate B of the optical signal is equal to the bit-rate of the encoded digital information and both are thus expressed in [bit/s] units. Exemplary optical bit-rates are 2.5, 10, 40 and 160 Gbit/s.
In the art, a general distinction is done between return to zero (RZ) and non-return to zero (NRZ) transmission. Independently from the modulation format, in RZ transmission the optical intensity of the optical signal always goes to a low intensity level between two adjacent pulses, while this does not happen in NRZ transmission. For the purpose of the present invention, “optical pulse”, or equivalently “optical symbol”, shall indicate the transmitted optical field, which solely occupies the time slot, or symbol-period, T and constitutes the elementary part of the transmitted optical stream, independently from the fact that the streamed optical field is pulsed or continuous.
In an attempt of increasing capacity of the optical telecommunication systems, modulation formats alternative to binary on-off key have been investigated. Among the alternative modulation formats, some phase shift key (PSK) techniques are particularly promising. These techniques encode information by modulating the optical phase of the carrier between a discrete set of M predetermined values. For example binary (M=2) phase shift key technique (BPSK) encodes a single bit in a time slot T by applying to the optical field in the time slot one phase value out of two predetermined phase values, which typically differ by π radians (0, π). Advantageously, M=2N in order to encode N bits of information in each transmitted optical symbol which is in a symbol-period T (multi-level or M-ary phase shift key—MPSK). The optical symbol rate B=1/T is expressed in [symbol/s] and the total transmission capacity [bit/s] is obtained by multiplying B by N. For example, in quaternary phase shift keying M=4 (N=2), the four phase symbols are typically in a quadrature constellation (quadrature phase shift keying—QPSK), as shown in FIG. 1. Here, X1, X2 is the phase symbol space and the depicted phase symbol values (+π/4, +3/4π, +5/4π, −π/4) are arbitrary. The choice of a reference system is arbitrary as it depends on the absolute optical field phase, which is a priori unknown. The modulation format is characterized by the distance between symbols. Thus any quadrature constellation may be arbitrarily chosen by rotation of the one depicted. For the purpose of the present invention, the term “MPSK”, or “multi-level phase shift keying”, will be referred to modulation formats having M greater than 2.
In optical differential multi-level phase shift key (DMPSK) techniques, information is encoded in the differential optical phase associated to successive symbols. For example, in DQPSK the four values of the optical phase differences between adjacent pulses are 0, +π/2, +π and +3/2π. Typically, a digital pre-coder is used to differentially encode two binary data streams each at a bit rate B [bit/s] and the resulting encoded signals are subsequently fed to an optical modulator so that a single transmitted optical stream at the same symbol rate B [symbol/s] is obtained. Decoding can be performed optically without the employment of a coherent local oscillator, by using a pair of unbalanced Mach-Zehnder interferometers (MZI). Advantageously, each MZI has one arm dimensioned in order to introduce an optical time delay equal to one symbol period with respect to the other arm. By setting the differential optical phase between the interferometer arms respectively to +π/4 and −π/4 and by employing balanced optical detectors (also known as differential photoreceiver) at the output of each interferometer, the DQPSK signal is converted back into two binary intensity sequences which represent the two original data streams at B [bit/s].
For each MZI, the output current Iout after the balanced photodetector is proportional to:
                                          I            out                    ÷                                    I              in                        2                          ⁢                  cos          ⁡                      (                          Δ              ±                              π                4                                      )                                              (        1        )            
where Iin is the input optical intensity into the MZI, Δ is the received phase difference between adjacent pulses and the sign plus or minus holds for MZI having differential optical phase between the interferometer arms equal to +π/4 and −π/4, respectively.
In order to perform a QPSK modulation, it has been proposed the use of a single phase-modulator driven by a four-level electrical input voltage in order to directly obtain the required four output phase levels. This solution has the disadvantage that commercially available phase-modulators need very high drive voltages Vπ. Besides that, it is necessary to drive the modulator with a voltage able to generate a 0 to 3/2π phase swing, feeding the modulator with more than Vπ, thus increasing drive electronics costs. In general, the “drive voltage”, Vπ, of a phase shifter is defined as the voltage which produces a phase shift of π at the optical carrier frequency.
Alternatively, it is known in the art the use of a cascade of two phase shifters, the first one producing a 0-π modulation, the second one a 0-π/2 modulation, or vice versa. Typically, a push-pull Mach-Zehnder Modulator (MZM) biased at the zero point and driven at a voltage equal to double the π voltage, Vπ, is used to apply the π-depth phase modulation. In case of a MZM, the π voltage, Vπ, is defined as the voltage which produces a phase shift difference between the first and second arm of π. A phase modulator consisting of a single waveguide with one electrode driven at half the π voltage may be used to apply the π/2-depth phase modulation.
In patent application WO 03/049331 it is disclosed a method and apparatus for encoding an optical signal having improved dispersion tolerance in a WDM optical communications system. There is provided a DQPSK modulator arrangement comprising a laser for producing an optical signal, which signal is split by a splitter, each part of the split signal being applied to a respective phase modulator, exemplarily a MZM. Each phase modulator is adapted to modulate the phase of the signal by 0 or π radians in dependence upon a respective drive voltage. The optical output of at least one modulator is passed through a phase shifter which applies a phase shift of π/2, such that the relative phase difference between the two parts of the split optical signals is ±π/2. A control electrode is used to provide the fine control. The split signals are recombined by an optical recombiner to form an optical PSK output. A further phase modulator is provided after the recombiner to chirp the optical PSK output Exemplarily, the further phase modulator applies a π/4 phase modulation to the output signal and it is driven by an oscillator which provides the clock rate corresponding to the data line rate. The oscillator must be synchronous with the data clock rate, i.e. it should be phase locked with the data stream.
Chirping relates to the variation of an optical signal's phase modulation, i.e. the optical phase is changed continuously within the symbol period in order to improve dispersion tolerance.
A generic interferometric modulator, such as for example a MZM, has an associated “extinction ratio” (“ER”) which is defined as the ratio between maximum and minimum optical intensity at the output of the modulator when operated in intensity modulation. Such extinction ratio typically depends not only on splitting ratios of input and output couplers of the interferometer but also on the rate at which the modulator is operated. For the purpose of the present invention, “Radio Frequency extinction ratio” (“RF-ER”) means the ER measured at high frequency modulation rate, i.e. higher than 1 Gbit/s, typically higher or equal than about 2.5 Gbit/s. For example, MZMs having two electrodes driven independently, known in the market as dual-drive MZMs (DD-MZM), typically show a Radio Frequency modulation ER which ranges between about 11 to 15 dB.
Applicant has found that a finite value of the ER of a modulator affects the MPSK optical signal emitted by the modulator with a phase error depending on the value of the ER.
For the purpose of the present invention, we will refer to a “non-ideal” or “finite” ER as a RF-ER equal to or less than about 30 dB. An “ideal” ER means a RF-ER greater than about 30 dB.
The Applicant has faced the problem of modulating an optical radiation in a multi-level phase shift key format while reducing the error in the phase of the transmitted optical symbols to an acceptable value. In particular, Applicant has faced the problem of reducing the optical phase error in a MPSK modulated signal due to the extinction ratio of the modulator employed for MPSK modulation. The Applicant has verified that the above problems are particularly relevant in the differential multilevel phase shift keying transmission, more particularly in the differential multilevel phase shift keying transmission employing a dual-drive MZM, and a need for better quality optical modulation is therefore strongly felt in these applications.